| Summary: | 碩士 === 國立成功大學 === 資訊工程學系碩博士班 === 94 === In the Public Key Inrfrastructure (PKI), certificate authority (CA) revokes public key by using public key certificate revocation. Some cryptosystems utilize an on-line server to revoke the public key certificate immediately. In this study, we firstly improve the efficiency of Public Key Revocation Free Encryption Scheme (PKRFE) that is one of the above-mentioned cryptosystems. The improved PKRFE saves sender the computing overhead of a public key verification formula in the encryption. Second, we propose the Certificate Verification Free Online Server Protocol (CVF-OLSP), in which, the sender utilizes the receiver’s public key without verifying the receiver’s public key certificate before encryption. The notion is that the sender encrypts message with the receiver’s and OLSP server’s public keys, hence the receiver needs the server’s help to decrypt ciphertext. Afetr the server authenticates receiver’s identity and public key, server forwards the necessary information to the receiver. However, the number of receivers is limited by online server’s computing ability. Hence, reducing the online server’s loading is an important problem. This study proposes an encryption system based on the notion of CVF-OLSP. Compares the proposed system with other on-line systems, the proposed scheme is more efficient in terms of compution complexity. Besides, we review other encryption systems and discuss their properties in this thesis.
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